The present invention relates to a retransmission control method and a transmitter in a wireless communication system, and more particularly to a retransmission control method and a transmitter in a wireless communication system in which a frame comprising data and control information necessary for reception processing of the data is transmitted and, when a reception failure is notified, the frame that failed to be received is retransmitted.
Present wireless communication, which is focused on high-speed large-volume communication, requires a technology for compensating the errors in a wireless transmission zone to improve the throughput. Among such technologies, a method based on repeat control (ARQ: Automatic Repeat reQuest) is often used. FIG. 19 illustrates the repeat mode implemented with the ARQ method. The ARQ is a method in which an error detection code such as CRC (Cyclic Redundancy Check) assigned to a transmission frame (packet) is used, error detection is conducted for each packet, if no error is present in the received packet, an ACK signal is returned to the transmission side, and a new packet transmission is requested. However, when the presence of an error is established, a NACK (Not-ACKnowledge) signal is returned and the same packet is requested to be repeated. In the figure, the RTT (Round Trip Time) is the time from the transmission of the initial packet to the repeat of this packet or transmission of the next packet.
A HARQ (Hybrid-ARQ) method is the development of the ARQ method (D. Chase, “Code Combining—A Maximum-Likelihood Decoding Approach for Combining an Arbitrary Number of Noisy Packets”, IEEE Trans. Commun., vol. 33, pp. 385-393, May, 1985). FIG. 20 is an explanatory drawing illustrating the HARQ method using a packet synthesis. With the HARQ method, after an error correction has been conducted, error detection is carried out for each packet by using, e.g., a CRC assigned to the tail of the transmission packet. If an error is detected, a NACK signal is returned to the transmission side and the repeat of the same packet is requested. At this time, the reception signal of the packet for which the error was detected is inputted in a buffer at the reception side. According to the HARQ method, the reception results of the repeated packet and the reception signal of the initially transmitted packet are synthesized. As a result, the reception characteristic can be further improved by using the gain of error correction coding.
FIG. 21 is a structural diagram of a signal modulation unit in a transmitter of a packet transmission system employing the HARQ method. FIG. 22 is a structural diagram of a signal demodulation unit in a receiver.
In the signal modulation unit of the transmitter shown in FIG. 21, a turbo coding unit 1a conducts turbo coding of transmission data. The coding ratio R of turbo coding is a constant, R=1/3. A punctured coding unit 1b attains the required coding ratio (for example, R=3/4) by using the prescribed puncturing pattern. The data modulation unit 1c conducts data modulation corresponding to the modulation method (multivalue modulation method). Generally, there are QPSK, 16QAM, and 64QAM modulation methods. The spread unit 1d multiplies the modulated signal by a spread code and spreads the signal correspondingly to a spread ratio.
The signal demodulation unit of the receiver shown in FIG. 22 is configured to conduct repeat synthesis prior to punctured decoding. A reverse spread unit 2a conducts a reverse spreading by multiplying the received signal by a reverse spread code identical to the spread code. Then, the data demodulation unit 2b carries out data demodulation corresponding to the modulation method. The repeat synthesis unit 2c, if a repeat packet data is received, conducts synthesis of this repeat packet data and the packet data with the same packet number that was previously received. As a result, a higher reception quality can be obtained. The punctured decoding unit 2d conducts punctured decoding corresponding to the coding ratio by using a puncturing pattern, and the turbo decoding unit 2e conducts turbo coding.
FIG. 23 shows an example of signal modulation in the signal modulation unit (FIG. 21). Here, the modulation method is 16 QAM and the coding ratio R is 3/4. The transmission data is denoted by A and 6-bit data A1-A6 is considered. If the coding ratio in turbo coding is taken as 1/3, the coded data become B1-B18. In the puncturing pattern PP corresponding to the coding ratio 3/4 , “1” appears in 8 bits of the 18 bits. Data B1-B7 and B16 corresponding to “1” of the pattern PP become the punctured coded data and are outputted as C1-C8 (rate matching). Because the original 6 bit data become 8 bit data, a coding ratio of 3/4 is attained. In data modulation, the 16 QAM modulation is executed, and E1, E2 data are produced. The data obtained by the data modulation is spread according to the spread ratio.
FIG. 24 shows an example of signal demodulation in the signal demodulation unit (FIG. 22). The processing flow herein is reversed with respect to that shown in FIG. 23. First, the received data are subjected to 16QAM demodulation and data C1-C8 are acquired. Then, punctured decoding is conducted by using the puncturing pattern PP. In this punctured decoding, data C1-C8 are written in positions corresponding to “1” in the puncturing pattern PP, and a turbo code with a coding ratio of 1/3 is obtained (derate matching). As a result, the original 6 bit data A1-A6 are decoded by conducting turbo decoding of the punctured decoded data.
FIG. 25 is the conventional example of a frame format used in repeat control of HARQ. FIG. 26 is an explanatory drawing illustrating the conventional frame repeat mode. One frame comprises a pilot PL, control information CNT, and data (including CRC) DT. The control information CNT serves for correct demodulation and decoding of the data at the receiving station and specifies the modulation method, coding ratio, puncturing pattern (PP), data length, spread ratio, antenna number (diversity), packet number, and number of repeat cycles. With the conventional method, if a repeat is requested from a receiving station, then a transmitting station modifies, if necessary, a portion of control information, such as the puncturing pattern (PP), packet number, or repeat cycle number, of the control information of the repeat frame and then carries out the frame repeat.
However, with the conventional retransmission control method, the control information that is the same during the new data transmission and during the repeat, for example, the modulation method, coding ratio, data length, and spread ratio is also repeated. As a result, because the control information that is not required to be repeated is resent, the unnecessary control information is present. In the next-generation mobile communication system, the delay has to be shortened and the packet transmission efficiency has to be increased by using packets with a short frame length. Therefore, if such unnecessary control information is present, the overhead of control information becomes large and the amount of data and pilot that can be transmitted in one packet is reduced. If the amount of data is reduced, the amount of transmitted information is decreased, thereby directly reducing the throughput. Furthermore, if the amount of pilot is decreased, then the channel estimation accuracy is decreased, decoding error occurs, and the number of repeat cycles increases. If the number of repeat cycles increases, the throughput also decreases. Therefore, in communication systems using packets with a short frame length, a retransmission control reducing the transmission of unnecessary control information is required to increase the throughput of the system and conduct high-speed transmission.
A technology has been suggested for conducting a repeat without interrupting the transmission (JP 03-262225A). In this conventional technology, the repeat is conducted by using an empty area of the transmission format and the unused channel and repeat processing can be conducted without stopping the transmission of information or reducing the transmission volume.
Furthermore, a technology has been suggested for increasing the throughput of the repeat transmission method (JP 2005-39726A). This conventional technology reduces the number of repeat cycles and increases the throughput by controlling an amount of transmission packet data that was requested to be repeated and accumulated in a buffer.
None of the conventional technologies relates to a retransmission control method for reducing the unnecessary transmission of control information.